1. Field of the Invention
This invention relates generally to LED circuits and in particular, providing an LED having a stable, highly accurate light output.
2. Description of the Related Art
FIG. 1 shows conventional light emitting diode (LED) circuit 100. LED circuit 100 includes at least light emitting diode 102, bipolar NPN transistor 104, sense resistor Rsense, and external reference voltage VREF. Light output LO of LED 102 is related to LED current ILED which, in turn, is an exponential function of diode voltage VD according to eq(1) below:ILED=ISeVD/(nVT)  Eq (1)
where:
IS is the reverse bias saturation current,
VD is the voltage across the diode,
VT is the thermal voltage,
and n is the emission coefficient.
Due to the exponential relationship between LED current ILED and diode voltage VD, a small change in diode voltage VD can result in a large change in LED current ILED and light output LO. Since there is essentially no base current (save for base leakage current which can for all purposes be ignored) in NPN transistor 104, ILED has essentially the same value as the current that flows through sense resistor Rsense according to eq(2) below:ILED≅(VREF−VBE)/Rsense  Eq (2)
Therefore, by using Rsense to control ILED, circuit 100 does not rely upon the exponential relationship between diode voltage VD and ILED (i.e., Eq (1)) to control light output LO but rather the linear relationship between ILED and Rsense (i.e., Eq(2)) since Rsense can easily be controlled to within <±1% with commonly available parts. However, VSENSE (VREF−VBE) is clearly dependent upon VREF and VBE and a dedicated external voltage reference can provide an accurate VREF having approximately ±3% regulation. However using the dedicated external voltage supply typically adds significant cost (that can be up to 2-4 times the cost of the LED itself). Thus to save cost, often, external voltage reference VREF is sourced at an digital output of a micro-controller. However, the associated variation in DC output voltage can be on the order of ±10%. Compounding the variability of the reference voltage supply VREF, NPN transistor 104 base emitter voltage VBE can have a part to part variance of about ±7%. All these variations taken together can result in substantial variability and inaccuracy of Vsense and thus the ILED (and light output LO). For example, using the topology of circuit 100, the overall accuracy in controlling ILED (and light output LO) with a dedicated external VREF of approximately 1.5 volts and VDD of approximately 3.3 V can be on the order of approximately ±20% for a desired current of 25 mA. This variability in ILED (and light output LO) can result in unacceptable variation in visual appearance of components that include these LEDs.
Another consideration is related to the use of LEDs in portable applications, such as laptop computers, where power consumption can be crucial to providing good battery life. In order to reduce overall power consumption, supply voltages have been trending down from, for example, 5.0 volts to 3.3 volts and lower. Therefore, it would be advantageous for Vsense to be as small a value as possible in order to minimize the required supply voltage according to equation (3A). Minimizing Vsense is also desirable to reduce the power Pc consumed (and wasted) by current ILED flowing through sense resistor Rsense according to Eq (3B):Vsupply=Vsense+Vce+VLED  Eq (3A)Pc=ILED×Vsense=Isense×Vsense  Eq (3B)
In order to achieve the minimal Vsense, Vref must be precisely set at a value according to Eq (4). From the equation, a typical Vref would be <1V. Dedicated external voltage reference capable of providing such low voltage is uncommon.Vref=Vbe+Vsense  Eq (4)
Therefore, providing a cost effective approach to providing a stable, precise, and accurate reference voltage in a low supply voltage environment is desired.